Devices of this type have operating frequencies ranging from several hundred MHz to several GHz, and are used in radio-frequency transmission circuits (mobile telephones, radio links, wireless data exchange, etc.), signal processing circuits, and sensor systems.
Most acoustic filters currently produced using BAW (Bulk Acoustic Wave) resonators are electrically coupled. However, some functions requiring electrical insulation between the input and output of the filter are still difficult to provide: they require the use of a balun, which is a costly arrangement in terms of overall dimensions and electrical losses. This drawback can be avoided by using acoustic coupling. In view of this situation, CRFs (Coupled Resonator Filters) such as those shown in FIG. 1 have been developed, as described in the following papers: G. G. Fattinger, J. Kaitila, R. Aigner and W. Nessler, Single-to-balanced Filters for Mobile Phones using Coupled Resonator BAW Technology, 2004 IEEE Ultrasonics Symposium, pp. 416-419, and J. D. Larson and R. C. Ruby, Thin-film acoustically-coupled transformer, U.S. Pat. No. 6,946,928 B2, September 2005. More precisely, a substrate S includes a stack of films in which the following are defined:                a Bragg mirror structure MR1;        a film of first piezoelectric material Piézo1 inserted between a first lower electrode Ei1 and a first upper electrode Es1;        a second Bragg mirror structure MR2; and        a film of second piezoelectric material Piézo2 inserted between a second lower electrode Ei2 and a second upper electrode Es2.        
These structures enable this requirement to be met by limiting the necessary surface area and the losses, but the technology required for their manufacture is highly complicated, as described in C. Billard, N. Buffet, A. Reinhardt, G. Parat, S. Joblot and P. Bar, 200 mm Manufacturing Solution for Coupled Resonator Filters, Proceedings of the 39th European Solid-State Device Research Conference (ESSDERC 2009), pp. 133-136.
In the field of acoustic filters, notably that of what are known as monolithic filters constructed on quartz piezoelectric material, the use of lateral coupling between resonators has already been proposed, with the resonators placed side by side as shown in FIG. 2. The piezoelectric material Piézo is inserted between two sets of upper and lower electrodes, indicated respectively by Es1, Ei1 and Es2, Ei2. Thickness shear (TS) waves are then excited, these waves being naturally evanescent outside the resonators, and by varying the distance between the resonators it is possible to vary the energy transmitted and thus modify the width of the pass band of the filter. In this case, the filter is known as a lateral coupling filter or monolithic filter. This works well for filters constructed on a piezoelectric substrate (notably quartz), but these resonators have a low resonant frequency (no more than a few hundred megahertz), owing to their considerable thickness, and they are not compatible with CMOS technologies (for mobile telephony, for example).
When these filters are transferred to thin films, with a thickness of the order of a millimeter, as described in J. Meltaus, T. Pensala, K. Kokkonen and A. Jansman, Laterally coupled solidly mounted BAW resonators at 1.9 GHz, 2009 Ultrasonics Symposium Proceedings, CMOS-compatible piezoelectric materials are used, for example aluminum nitride (AlN) or zinc oxide (ZnO). However, these materials naturally have acoustic characteristics such that the waves propagated therein are not necessarily evanescent outside the resonators. A structure such as that of FIG. 2 therefore produces very strongly coupled resonators, regardless of the distance between them, resulting in behavior identical to that of a single resonator.
However, the acoustic behavior of the stack can be modified by adding other materials to the stack, or by using the perturbation effect created by a Bragg mirror, thus restoring propagation conditions similar to those found naturally in quartz, as described in G. G. Fattinger, S. Marksteiner, J. Kaitila and R. Aigner, Optimization of acoustic dispersion for high performance thin-film BAW resonators, 2005 IEEE Ultrasonics Symposium Proceedings, pp. 1175-1178.
This is the solution which is described, for example, in J. Meltaus, T. Pensala, K. Kokkonen and A. Jansman, Laterally coupled solidly mounted BAW resonators at 1.9 GHz, 2009 Ultrasonics Symposium Proceedings.
In this case, the distance between two laterally coupled resonators, corresponding to the evanescence length, is very small, being typically of the same order of magnitude as the thickness of the film. It then becomes difficult to achieve precise control of the distance between the resonators, and consequently the coupling, which has a direct effect on the width of the filter pass band.